1. Field of the Invention
The present invention relates to a toner composition. More particularly, the present invention relates to a toner composition containing an external additive and used in an electrophotography image forming apparatus such that a stable charge and a stable charge distribution are retained in spite of environmental variation and periodic variation due to a long use of image printing.
2. Description of the Related Art
As an electrophotography image forming apparatus, there are, for example, a facsimile, a laser beam or light emitting diode (LED) printer, a laser beam copier, a variety of electrophotography output apparatuses and the like. Such an electrophotography image forming apparatus includes a dry type electrophotography image forming apparatus using a dry toner and a wet type electrophotography image forming apparatus using a wet toner. The present invention mainly relates to a dry type electrophotography image forming apparatus, although it is not strictly limited thereto.
A dry toner generally includes a binder resin which contains a colorant, a charge control agent, and other additives. The additives may be classified into internal additives added to an inside of the toner particle and external additives added to a surface of the toner particle, and are generally added to enhance the functionality of the toner.
The colorants may be classified into dye type colorants and pigment type colorants. The pigment type colorants are generally used because they are advantageous with respect to thermal stability and resistance against light compared with the dye type colorants.
The charge control agents are added to control a charge of the toner particle. The charge control agents are added differently depending on the charge type, i.e., positive (+) or negative (−) type of the toner particles.
Among the functional additives added to the toner, the release agent is utilized to enhance the release property. The release agent enhances the release property between the roller and the toner when a toner image is transferred onto and fixed on a record medium, thus preventing toner offset and also preventing the record medium from adhering to the roller due to the toner and generating a jamming phenomenon.
FIG. 1 schematically illustrates a dry type electrophotography image forming apparatus. In particular, FIG. 1 illustrates a dry type electrophotography image forming apparatus using a non-contact developing method. Referring to FIG. 1, after a photosensitive body 100 is charged using a charge unit 600, an image is exposed using a laser scanning unit 900 to form a latent image on the photosensitive body 100. Toner 400 is fed to a developing roller 200 by a supply roller 300. The toner fed to the developing roller 200 forms a thin and uniform toner layer on a surface of the developing roller 200 by a toner layer control unit 500, and at the same time, is contacted by both the developing roller 200 and the toner layer control unit 500. The toner that has passed through a member of the toner layer control unit 500 is developed in the form of a latent image formed on the photosensitive body 100. The developed toner is transferred onto a record medium by a transfer roller (not shown) and is then fused by a fusing unit (not shown). After the transfer of the toner, the toner remaining on the photosensitive body 100 is cleaned off the photosensitive body 100 by a cleaning blade 700. The cleaned off toner 800 is collected in a bin near the cleaning blade 700.
In such a dry type electrophotography image formation, the toner has fine particles which are a few micrometers in size, and is used to form a print image on a record medium. A charge property and flow properties of the toner are highly significant factors controlling quality of the print image. The print image is required to maintain stable quality even after an extended time has elapsed as well as in an initial printing. Also, it is required that the variation in the print image quality be minimized under a condition of an environmental variation. To satisfy the requirements specified above, the toner may include a variety of internal or external additives. By using such internal or external additives, it is possible to control the charge properties and the flow properties of the toner. However, the external additives, which are added for the purposes such as stabilization of a toner charge, prevention of a fog, and enhancement of a development efficiency, have a limitation in improving the image quality. In other words, the external additives have a disadvantage in that a large variation in the toner charge characteristic is generated by an environmental variation. For example, overcharge is easily generated under a low temperature and a low humidity environment. Fog, toner dispersion or the like is generated in a non-image region due to a lowering of charge under a high temperature and a high humidity environment. Also, in an initial stage of the printing, the external additives have a uniform charge and charge distribution, but when external additives are left without any management, the charge is lowered remarkably. Further, when images are printed for a long time, charge is reduced, charge distribution is non-uniform, image concentration is lowered, and fog and toner dispersion are caused.
In particular, to provide the toner with a uniform charge property, it is necessary to form a thin toner layer on the developing roller. However, when a thin toner layer is formed, the toner degenerates due to toner stress, or the development efficiency is abruptly lowered, so that image concentration may be easily decreased. Also, when the toner charge is lowered to improve the decreased development, the fog may be increased or contamination may be caused due to the toner dispersion.